1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus applicable for developing an electric latent image or a magnetic latent image. More particularly, the invention relates to an image forming method and an image forming apparatus apprawhich improves the service life of a developer and gives a stable image concentration.
2. Description of the Related Art
There is conventionally known a method of converting an electrostatic latent image into a sensible image by bearing a dry type developer serving as an image developing agent on the surface of a developer bearing member, transferring and supplying the developer to the proximity of the surface of a latent image bearing member bearing an electrostatic latent image, and developing the electrostatic latent image while applying an alternate electric field between the latent image bearing member and the developer bearing member.
The aforesaid developer bearing member, often taking the form of a developing sleeve, will hereinafter be referred to as the "developing sleeve", and the latent image bearing member, often implemented in the form of a photosensitive drum, will hereinafter be called the "photosensitive drum".
A conventionally known method of development includes those called the magnetic brush developing processes (for example, disclosed in Japanese Patent Laid-Open No. 55-32,060 and No. 59-165,082) comprising the steps of forming a magnetic brush on the surface of a developing sleeve having a magnet arranged therein, using a two-component type developer consisting of, for example, magnetic carrier particles and non-magnetic toner particles, bringing this magnetic brush into sliding contact with, or near, a photosensitive drum arranged opposite thereto with a slight development gap in between, and applying continuously an alternate electric field between the developing sleeve and the photosensitive drum, thereby causing displacement and reverse displacement of toner particles from the developing sleeve side to the photosensitive drum side. In the foregoing two-component magnetic brush developing process, toner in an amount corresponding to the amount of toner consumed by development is supplied, thereby keeping a constant mixing ratio of toner particles to magnetic carrier (hereinafter simply referred to as the "T/C ratio"). Various techniques have conventionally been proposed for the detection of the T/C ratio in the developing vessel. A technique, for example, comprises the steps of providing detecting means around a photosensitive drum, irradiating a light onto toner having displaced from the side of a developing sleeve to the photosensitive drum side, and determining a T/C ratio from the transmitting light and the reflected light at this point; one comprising the steps of providing detecting means on a developing sleeve, and determining a T/C ratio from the reflected light when irradiating a light onto a developer coated on the developing sleeve; and another one comprising the steps of providing a sensor in a developing vessel, detecting a change in magnetic permeability (.mu.) of a developer within a certain volume near the sensor by the utilization of coil inductance, thereby determining a T/C ratio. These techniques have been proposed and practically applied.
However, the technique of detecting the T/C ratio from the amount of toner on the photosensitive drum has a problem in that, along with the recent downsizing tendency of copying machines and image forming apparatus, a space for installing detecting means cannot be ensured. The one for detecting the T/C ratio from the reflected light upon irradiating the light to the developer coated on the developing sleeve is defective in that, when detecting means is stained by toner splash or the like, the T/C ratio cannot accurately be detected. In contrast, in the technique of detecting a change in magnetic permeability (.mu.) of the developer within a certain volume near the sensor by the utilization of the coil inductance to determine the T/C ratio (hereinafter referred to as the "toner concentration detecting sensor"), the sensor alone is available at a low cost, and the machine is free from the problems of installation space or stain by toner splash. In a copying machine or an image forming apparatus having only a limited space for installation, of a low cost, this would be the optimum T/C ratio detecting means.
In the toner concentration detecting sensor using a change in magnetic permeability of the developer, a larger magnetic permeability means a decrease in T/C in the developer within a certain volume, and hence a decrease in the amount of toner in the developer. Supply of toner is therefore started. A smaller magnetic permeability means, on the other hand, a higher T/C in the developer within a certain volume, and hence an increase in the amount of toner in the developer. Supply of toner is therefore discontinued. T/C is thus controlled in accordance with such a sequence.
In the toner concentration detecting sensor detecting a change in magnetic permeability (.mu.) of the developer within a certain volume as described above, however, a change in bulk density of the developer itself under the effect of some cause or other leads to a change in magnetic permeability of the developer. This is associated with a defect of this sensor in that the sensor output shows a change corresponding to the change in magnetic permeability. In other words, a change in bulk density in the developing vessel in spite of a constant T/C in the developing vessel results in a change in the amount of the developer (carrier) within the certain volume near the toner concentration detecting sensor. The change in magnetic permeability therefore inevitably results in a change in the sensor output. As a result, a sensor output showing a decrease in the amount of toner is issued although toner is not consumed, and toner is supplied. Or, although the amount of toner decreases, a sensor output showing no decrease in toner is issued, and toner is not supplied. The former case poses problems of the image density increased by the over-supply of toner, overflow of the developer from the developing vessel as a result of increase in the amount of developer brought about by the increase in the amount of toner, and toner splash caused by a decrease in the charge amount of toner along with the increase in toner ratio in the developer. The latter case causes, on the other hand, image deterioration or a lower image density resulting from the decrease in the amount of toner in the developer, or a lower image density resulting from an increase in the charge amount of toner.
A detailed study carried out by the present inventors revealed that these problems were caused mainly the following three phenomena in the system comprising the developing machine and the developer used in the foregoing developing process.
The first phenomenon is caused by crushed toner conventionally used in common. Since individual particles of crushed toner have irregular surfaces and are different from each other, bulk density of the developer tends to vary between states thereof including stationary, flowing and holding states. Variation of bulk density caused by a change in the toner shape through use for a long period of time is particularly large.
The second phenomenon is caused by a configuration in which, in order to prevent non-uniform coating of the developer on the developing sleeve, the developer is accumulated in the proximity of the regulating blade of the developing sleeve to compress the developer. In this configuration, the developer is slowly compressed mechanically and magnetically, resulting in a change in toner shape which in turn leads to a change in bulk density of the developer, or in a change in bulk density caused by buried external additive, and these changes cause changes in magnetic permeability of the developer.
The third phenomenon is a problem regarding a change in charge amount of toner in the rotation of the developing sleeve. Because the developer is liable to be compressed in a developer sump near the regulating blade of the developing sleeve as described above, there is an increase in frictional force between particles of developer along with the rotation of the developing sleeve. According as the developing sleeve rotates more times, the external additive on the toner tends to transfer to the carrier more easily, thus resulting in a larger change in toner charge amount. A larger change in toner charge amount suggests a larger change in repulsion between particles of the developer. A larger toner charge amount causes a stronger repulsion between developer particles, and a resultant larger distance between particles of the developer in turn causes a decrease in bulk density of the developer. Since bulk density of the developer largely varies under the effect of these three phenomena, it has been difficult with the conventional configuration of developing machine and developer to fully utilize a toner concentration detecting sensor based on the change in magnetic permeability.